1. Field of the Invention
The present invention relates to resistance welding, and more particularly to an improvement in the resistance welding property of zinc plated steel plates.
2. Description of the Prior Art
Zinc plated steel plates, having at least one surface covered with a plating layer composed of mainly zinc, are widely used as a material for an automobile or a washing machine body. The zinc plated steel plates are generally joined by electric resistance welding such as spot welding, projection welding, and seam welding, but they have poor electric resistance welding properties compared with ordinary steel plates having no plating layer, because insufficient heat is generated on mating surfaces of the zinc plated steel plates. This results from the fact that zinc has a relatively low electric resistance value and the electric resistance value of the mating surfaces decreases because zinc is soft and provides good affinity between the zinc plated layers when zinc plated steel plates to be joined are pressed against each other. Accordingly, weld current and weld time for spot welding the zinc plated steel plates are increased by 25 to 50% and 50% to 100%, respectively, compared with the ordinary steel plates. Therefore, the electric power consumption increases.
In addition, the size of a weld nugget formed by welding varies, and the weld strength becomes unstable, and heat generation on the contacting surfaces of electrodes and the steel plates increases so that the electrodes are damaged badly. Further, zinc adhered to the electrodes should be removed frequently. Thus, the productivity becomes worse, and the manufacturing costs increase.
Since the anti-corrosion property of the zinc plates steel plates depends on the sacrificing corrosion of zinc, it is preferred to improve the anti-corrosion property by increasing the thickness of the zinc plating layer. When the thickness of the zinc plating layer is increased, the resistance welding property is further aggravated by the adverse effects mentioned above.
Consequently, as far as the material for automobile body is concerned, many zinc plated steel plates with less zinc adherence, sacrificing the anti-corrosion property, are used currently except for automobiles exported to north America and north Europe where snow-melting agents are spreaded on the roads. And the following are also used: alloyed zinc plated steel plates, zinc alloy plated steel plates, and multi-layered zinc plated steel plates. These are improved to provide better anti-corrosion property with relatively thin plating layer.
Although the zinc plated steel plates with thinner plating layer usually have good resistance welding property, their resistance welding property should further be improved because it is far inferior to that of bare steel plates without plating layer. The improvement of the resistance welding property is desired especially for the zinc plated steel plate with much zinc adherence, which is superior in anti-corrosion property, used in automobile bodies exported to north America and north Europe where superior anti-corrosion property is required.